Lasers and light emitting diodes (LEDs) are used as sources of blue light in various fields of optoelectronics, such as optical measurement, optical transmission and optical displays. Light-emitting devices which use LEDs (particularly those which emit blue light) utilizing GaN semiconductors are known (see, e.g., refer to S. Nakamura, T. Mukai and M. Senoh: Jpn. J. Appl. Phys., Vol. 30 (1991) L1998). However, since the line width of LED light is wide (a single wavelength cannot be created), lasers have in recent years been more widely used than LEDs in the field of optoelectronics.
For example, with some ZnCdSSe semiconductor lasers, an acceptable blue light output is obtainable (see, e.g., M. A. Hasse, J. Qiu, J. M. DePuydt and H. Cheng: Appl. Phys. Lett., Vol. 59 (1991) 1272). Nevertheless, under the present circumstances such devices can only be used upon cooling to extremely low temperatures, and therefore a practical light output cannot be obtained at a room temperature.
In addition, light-emitting devices that introduce high-power solid laser light or high-power semiconductor laser light into non-linear optical crystals (e.g., dielectric substances such as LiNbO.sub.3 and KNbO.sub.3, semiconductors such as GaAs, organic substances, etc.) to generate a second harmonic are known (e.g., refer to A. Yariv: Introduction to Optical Electronics, 4th ed.; Saunders College Publishing, (Holt, Rinehart and Winston, 1991)). As shown in FIG. 6, in this type of device a laser source 23 and non-linear optical crystals 24 are arranged between a pair of optical reflectors 21, 22, and laser light is launched through the non-linear optical crystals 24 to generate a second harmonic, and blue light is extracted from the reflector 22 which has the higher transmission of the second harmonic. However, the larger the size of the device, the greater the cost of its production, and since it is composed of a combination of multiple components, its problems include extremely difficult control and unstable output.
Further, devices are known which extract a second harmonic from the end surface of normally striped GaAs or AlGaAs semiconductor lasers (see, e.g., N. Ogasawara, R. Ito, H. Rokukawa and W. Katsurashima: Jpn. J. Appl. Phys., Vol. 26 (1987) 1386), but the power of the fundamental wave inside these devices is low due to a low end facet reflectivity. Also, the absorption loss is large due to the long cavity. These device have even greater difficulty in achieving a structure for phase matching. These disadvantages make it impossible to generate the second harmonic with high efficiency.
Further thought has been directed towards extraction of a second harmonic in the direction perpendicular to the cavity. See, e.g., D. Vakhshoori, R. J. Fisher, M. Hong, D. L. Sivco, G. J. Zydzik, G. N. S. Chu and A. Y. Cho: Appl. Phys. Lett., Vol. 59 (1991) 896. However, in a device of the type disclosed in this publication, the output power of the second harmonic is small and the emitted light is distributed over a wide range, therefore condensing of the light is difficult; thus, at present, a practical application for this device has not yet been achieved.